1. Field of the Invention
The present invention relates to a metal nanoparticle composite applicable to various devices that use Local Surface Plasmon Resonance (LSPR) and a production method of the metal nanoparticle composite.
2. Description of Related Art
LSRP is a resonance phenomenon due to an interaction between electrons in metal nanoparticles or metal microstructures having a size of several nanometers to 100 nm and light of a specific wavelength. Since a long time ago, LSPR has been used in stained glass which exhibits bright colors by mixing metal nanoparticles in a glass. In recent years, in the industry, researches have been conducted on developing high-power light-emitting lasers, which utilize enhanced light intensity and on applications such as biosensors, which utilize the resonance state change property when molecule bonding occurs.
In order to apply such LSPR of metal nanoparticles to sensors, metal nanoparticles should be immobilized in a matrix such as a synthetic resin. However, if the metal nanoparticles are on a nanometer scale, the aggregation and dispersion characteristics change, for example, the stable dispersion due to an electrostatic repulsion effect becomes difficult, and aggregation is likely to occur. Therefore, for plasmon devices that use LSPR, it is essential to homogenously disperse the metal nanoparticles in the matrix.
For example, the following Patent Documents 1 to 6 propose technologies of metal nanoparticle composites for immobilizing metal nanoparticles in matrixes, such as resin. In Patent Document 1, with a high-elastic modulus polymer composite material due to a small particle size, a good dispersion characteristic of particles and good adherence of particles to the matrix, a polymer-metal cluster composite with improved elastic modulus formed by filling metal particles in a thermoplastic or thermosetting polymer matrix is disclosed, where particle diameters of the metal particles are 10 angstroms to 20 angstroms, and the metal particles are uniformly dispersed with a volume fraction of 0.005% to 0.01%. However, in the polymer-metal cluster composite in Patent Document 1, metal nanoparticles are dispersed to improve the elastic modulus, and as a result, the particle diameters are too small to be applied in generating plasmon resonance.
Patent Document 2 has disclosed a production method of a metal nanoparticle dispersion for forming a novel conductive coating that can substitute the electroless plating method or applied to a granular magnetic film by reducing nanoparticle dispersion in a gaseous phase after a resin substrate containing an ion exchange site contacts with a solution containing metal ions. In this method, during the hydrogen reduction, metal ions are dispersed in the resin while reacting; therefore, no metal nanoparticle is present in a region from the surface of the resin substrate to a depth of tens of nanometers (which is 80 nm in the example of Patent Document 2). Although this feature provides an advantage that no protection film is needed when the magnetic film is formed by using magnetic nanoparticles because the metal nanoparticles are embedded in the depth of the matrix, the feature sometimes becomes a disadvantage when being implemented. In addition, as disclosed in Patent Document 2, through the heat-reduction in a hydrogen atmosphere, the metal nanoparticles precipitated through reduction become a catalyst for promoting thermal decomposition of the resin matrix caused by hydrogen, and sometimes contraction of the resin matrix occurs. In addition, control of the particle spacing between adjacent metal nanoparticles in the resin matrix is not taken into consideration.
Patent Document 3 has disclosed the following method: contacting a polyimide resin film with a liquid containing metal ions to dope metal ions in the resin film, where the polyimide resin film has been processed by an alkaline aqueous solution to introduce a carboxyl group into the polyimide resin film; after that, at a temperature higher than the reduction temperature of the metal ions, performing a first heat treatment in a reducing gas, so as to form a layer which is a polyimide resin dispersed with metal nanoparticles; and further performing a second heat treatment at a temperature different from that of the first heat treatment. Patent document 3 has documented that the volume fill ratio of the metal nanoparticles in the composite film may be controlled by adjusting the thickness of the metal nanoparticle dispersion layer through the second heat treatment. Patent Document 3 has documented the following situation: through the heat treatment in the reducing gas, metal ions that are bonded or adsorbed in a region from a surface of the polyimide resin film to a depth of several micrometers are dispersed in the resin film while participating in the reduction reaction. Therefore, the metal nanoparticles are uniformly dispersed in the resin matrix within a region from the surface of the resin film to a depth from tens of nanometers to several micrometers, and no metal nanoparticle is present near the surface. Like the feature in Patent Document 2, this feature also becomes a disadvantage sometimes when being implemented. In addition, similar to the technology in Patent Document 2, the technology in Patent Document 3 does not provide the consideration of controlling of the particle spacing between adjacent metal nanoparticles in the resin matrix.
Patent Document 4 and Patent Document 5 have disclosed a heat sensitive color-developing element using a solid matrix, wherein micro metal nanoparticles that irreversibly grow by means of temperature change are dispersed in the solid matrix through plasmon resonance. The technology of Patent Documents 4 and 5 is under the premise that the micro metal nanoparticles are aggregated by means of temperature change, and the particle diameter is increased to generate plasmon attraction; metal nanoparticles are dispersed into the matrix, while controlling over the spacing between adjacent nanoparticles is not taken into consideration.
Patent Document 6 has disclosed a method for photo-reducing a metal precursor through exposure to ultraviolet rays after the metal precursor is dispersed into a matrix of polymer substance at a molecular level during a process of dispersing metal particles into the polymer matrix, so as to solve problems such as intermiscibility with the polymer matrix, interface defects and cohesion among particles. However, in the method of Patent Document 6, the metal nanoparticles are precipitated through reduction by using ultraviolet rays, so due to the influence of an ultraviolet radiation surface, a gradient concerning precipitation density of the metal nanoparticles is generated from a surface portion to a deep portion of the matrix. That is, from the surface portion to the deep portion of the matrix, the particle diameters and the fill ratio of the metal nanoparticles tend to decline consecutively. In addition, although the particle diameters of the metal nanoparticles obtained through photo-reduction are the greatest at the ultraviolet radiation surface, namely, the surface portion of the matrix, the particle diameters are about 10 nm at most. In addition, it is difficult to disperse metal nanoparticles having the same particle diameters or larger particle diameters into the deep portion.